January 22, 2010

Haiti - liquefaction

Much has been written on the chaos and the tragedy of Haiti while I have been
offline, but I thought it worthwhile highlighting a couple of excellent sources.
Chris Rowan at Highly Allochthonous has put together a very helpful summary
of Caribbean seismic activity and tectonics - highly recommended.

Last October, on the twentieth anniversary of the Loma Prieta earthquake in
California, I wrote on the devastating
role of liquefaction of unconsolidated sand during that event and other
earthquakes. In Haiti, tragically, the effects are there to see again. Dave
Petley, of Durham University, who writes the fascinating Dave's Landslide
Blog, has put together a series of posts using Google Earth
high-resolution imagery to illustrate liquefaction-triggered damage. These can
be found here,
here,
and here.

The high-resolution imagery made freely available through Google Earth is
extraordinary - of vital use to the relief effort, and simply riveting for
everyone else. Combined with the latest historical imagery facility, Google
Earth now allows "before and after" viewing of this kind of event, as well as
the general ability to observe geologic and landscape changes - I can see a high
risk of becoming unduly obsessed with this, with a deleterious effect on time
management.

I looked more closely at a couple of examples of liquefaction damage to
harbour facilities that Dave had highlighted (and it's no wonder that the news
reports the enormous problems associated with delivery of anything by sea). As
is so often the case, large parts of harbours are built on manmade land created
by artificial landfill, the perfect material for liquefaction. In the image at
the head of this post, the effects of sand movement - compaction and lateral
spreading under liquefaction - can be clearly seen, long fissures and associated
slumping (the image was clearly taken at low tide, but I would suspect that the
sandy coloured material is not a beach, but artificial fill - it certainly
wasn't there in images from previous years.

Dave points out a CNN
video report from one of the harbour locations that he highlights. It's
well-worth watching, in part for the material on which the reporter is initially
walking - it looks like sand. And, in the imagery below, the sandy-coloured
smears are clearly associated with the massive liquefaction-induced fractures,
and I suspect that this is liquefied sand ejected from those fractures, just as
I described for the Loma Prieta earthquake.

[Breaking news - tonight, February 2, on UK television, a progamme on the causes of the earthquake followed the first geologist on the ground and, standing at exactly the scene below, he picked up some white sand and described how it had been expelled from the fissures by liquefaction.]

And then, googling along the coast, away from urban devastation, I noticed
the delta shown in the "before and after" images below. Over just a few years,
there are dramatic changes to the entire shape of the delta and the architecture
of its channels. Some of this was undoubtedly happening anyway, in part under
the influence of expanding agriculture, but I couldn't help but be impressed by
the now dramatically braided channel system, the two massive and
dominant channels, and the amount of sediment sluicing through them, one branch
creating a great sediment plume.

Are we seeing increased sediment charge from earthquake-induced landsliding
in the interior? How much of the changes in channel architecture may have been
the result of the earthquake? Clearly, I don't have the answers, but the imagery
raises some fascinating questions.